Ultrasonic transducers have been used in a number of sensing applications such as a medical imaging non-destructive evaluation, gas metering and a number of ultrasound generating applications such medical therapy, industrial cleaning, etc. One class of such transducers is the electrostatic transducer. Electrostatic transducers have long been used for receiving and generating acoustic waves. Large area electrostatic transducer arrays have been used for acoustic imaging. The electrostatic transducer employs resilient membranes with very little inertia forming one plate of an electrostatic transducers support above a second plate. When distances are small the transducers can exert very large forces. The momentum carried by approximately half a wavelength of air molecules is able to set the membrane in motion and vice versa. Electrostatic actuation and detection enables the realization and control of such membranes. Alternatively the membranes can be actuated using piezoelectric and magnetic transducers.
Broad band microfabricated capacitive ultrasonic transducers (cMUTs) may include multiple elements including identical or different size and shape membranes supported above a silicon substrate by walls of an insulating material which together with the membrane and substrate define cells. The walls are formed by micromachining a layer of insulation material such as silicon oxide, silicon nitride, etc. The substrate can be glass or other substrate material. The capacitive transducer is formed by a conductive layer or the membrane and conductive means such as a layer either applied to the substrate or the substrate having conductive regions. In other types of broadband ultrasonic transducers in which the membranes are actuated by piezoelectric transducers (pMUTs) the cell walls need not be made of insulating material.
The fabrication of capacitive micromachined ultrasonic transducers has been described in many publications and patents. For example U.S. Pat. Nos. 5,619,476; 5,870,351 and 5,894,452, incorporated herein by reference, describe the fabrication of capacitive or electrostatic type ultrasonic transducers in which the membranes are supported above a substrate such as silicon by insulative supports such as silicon nitride, silicon oxide or polyamide. The supports engage the edges of each membrane to form a cell or cells. A voltage applied between the substrate and conductive film on the surface of the membranes causes the membranes to vibrate and emits sound, or in the alternative, received sound waves cause the membranes to vibrate and provide a change in capacitance. The membranes can be sealed to provide operation of the transducers immersed in liquids. Generally the transducers include a plurality of cells of the same or different sizes and/or shapes. In some applications the multi-cell transducer elements are disposed in arrays with the electrical excitation of the elements controlled to provide desired beam patterns. The same technology can be employed to fabricate pMUTs and mMUTs.
Generally the membranes in the prior art cMUTs are grown or deposited on an insulating film and the insulating film is selectively etched through openings in the membrane to provide underlying cavities. Membrane properties which depend upon the process parameters and the predictability, reproducibility and uniformity of the membranes are compromised. Further the formation of membranes with underlying cavities requires complex processing steps. Furthermore it is difficult to generate complex cavity membrane structures using the conventional MUT fabrication technology of the prior art.